1. Field of the Invention
This invention relates to high pressure discharge lamps and relates in particular to short arc type high pressure discharge lamps suited for use in liquid crystal projectors etc.
2. Description of Related Art
In this kind of high pressure discharge lamps, electrode rods of a pair of electrodes arranged oppositely to each other in the light emitting part of the discharge vessel are buried and sealed in sealing parts at both ends of the light emitting part. For said sealing parts there is the problem of a damage or breakage of the quartz glass caused by the difference in the coefficients of thermal expansion of the electrode rod made from normal tungsten and the sealing part made from quartz glass, and this problem becomes even more severe with the above mentioned high pressure discharge lamps for use in projectors, as a large amount of mercury of at least 0.15 mg/mm3 is contained in the light emitting part and the mercury vapor pressure at the time of switching-on becomes at least 100 atm.
To solve this problem, various measures have been taken to buffer the difference in the thermal expansions between the electrode rod and the sealing part, and as one of these it was suggested in JP-A-2008-529252 and corresponding US 2008/0185950 A1 to provide axially directed grooves in the electrode rods. This known technique is shown in FIG. 5. In this figure, electrodes 1 made from tungsten comprise tip end parts 2 arranged oppositely to each other in a light emitting part 5 and electrode rods 3 at which said tip end parts 2 are mounted. The electrode rods 3 are embedded and sealed in sealing parts 6 connected with the light emitting part 5 of a discharge vessel 4 made from quartz glass. At said electrode rods 3, axially directed grooves 31 are formed, and the part in which these grooves 31 are formed is embedded and sealed in the sealing part 6. By doing so, the surface roughness in the circumferential direction of the electrode rod 3 becomes higher than the surface roughness in the longitudinal direction, and the flaw of a breakage of the sealing part caused by a difference in the coefficients of thermal expansion between the material of the electrode rod 3 (tungsten) and the material of the sealing part 6 (quartz glass) can be eliminated.
But this known technique results in the inconvenience that the mechanical strength decreases and the phenomenon of bending or breaking of the electrode rod 3 occurs since the diameter of the groove bottom circle defined by the bottom parts of the grooves 31, which determines the mechanical strength of the electrode rod 3, becomes smaller than the diameter of the electrode rod 3 because of the formation of the grooves 31 in the electrode rod 3.